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Diverse sources of hippocampal unitary inhibitory postsynaptic potentials and the number of synaptic release sites
Abstract
Dual intracellular recordings from microscopically identified neurons in the hippocampus reveal that the synaptic terminals of three morphologically distinct types of interneuron act through GABAA receptors. Each type of interneuron forms up to 12 synaptic contacts with a postsynaptic principal neuron, but each interneuron innervates a different domain of the surface of the postsynaptic neuron. Different kinetics of the postsynaptic effects, together with the strategic placement of synapses, indicate that these GABAergic interneurons serve distinct functions in the cortical network.
... https://doi.org/10.1038/s41593-023-01380-x Perisomatic inhibition from PV-INs provides powerful inhibitory control over the principal neuron population [29][30][31][32] . To further examine the role of PV-IN perisomatic inhibition in local neuronal activity during motor control, we abolished presynaptic GABA release from PV-INs by expressing Cre-dependent TeTxLC virus in PV-Cre mice (PV-TeTxLC mice) and monitored the M2 neuron activity (Fig. 3b). ...
... The abolishment of perisomatic inhibition in PV-TeTxLC mice excessively increased the overall activities of M2 neurons, consistent with the notion that PV-INs are critical regulators of cortical E/I balance 33,34 . Perisomatic inhibition from PV-INs provides powerful inhibitory control over the PyN population [29][30][31][32] . Fast-spiking PV basket cells (PV-BCs) are well suited for regulating the balance, gain and network oscillation of relatively broad PyN populations 47,48 . ...
Genetically defined subgroups of inhibitory interneurons are thought to play distinct roles in learning, but heterogeneity within these subgroups has limited our understanding of the scope and nature of their specific contributions. Here we reveal that the chandelier cell (ChC), an interneuron type that specializes in inhibiting the axon-initial segment (AIS) of pyramidal neurons, establishes cortical microcircuits for organizing neural coding through selective axo-axonic synaptic plasticity. We found that organized motor control is mediated by enhanced population coding of direction-tuned premotor neurons, with tuning refined through suppression of irrelevant neuronal activity. ChCs contribute to learning-dependent refinements by providing selective inhibitory control over individual pyramidal neurons rather than global suppression. Quantitative analysis of structural plasticity across axo-axonic synapses revealed that ChCs redistributed inhibitory weights to individual pyramidal neurons during learning. These results demonstrate an adaptive logic of the inhibitory circuit motif responsible for organizing distributed neural representations. Thus, ChCs permit efficient cortical computation in a targeted cell-specific manner.
... [32][33][34] The bistratified cell (BiC) is defined by axons limited to stratum radiatum (SR) and SO. 34,35 These two interneuron types have been reported to co-express SOM and PV 22,36 ( Figure 1). The other two stereotypic PV interneurons in the hippocampus are the AAC cells (also known as chandelier cells), targeting PC axon initial segments, 37,38 and the FS BCs, targeting PC somata and proximal dendrites. ...
One of the most captivating questions in neuroscience revolves around the brain's ability to efficiently and durably capture and store information. It must process continuous input from sensory organs while also encoding memories that can persist throughout a lifetime. What are the cellular-, subcellular-, and network-level mechanisms that underlie this remarkable capacity for long-term information storage? Furthermore, what contributions do distinct types of GABAergic interneurons make to this process? As the hippocampus plays a pivotal role in memory, our review focuses on three aspects: (1) delineation of hippocampal interneuron types and their connectivity, (2) interneuron plasticity, and (3) activity patterns of interneurons during memory-related rhythms, including the role of long-range interneurons and disinhibition. We explore how these three elements, together showcasing the remarkable diversity of inhibitory circuits, shape the processing of memories in the hippocampus.
... Hippocampal inhibitory interneurons (INs) are heterogeneous populations of GABAergic inhibitory cells with varied morphological, molecular, and electrophysiological properties, as well as specialized network functions [7][8][9][10][11][12][13][14]. In CA1 hippocampus, somatostatin-expressing interneurons (SOM-INs) are a major subgroup of INs which include Oriens-Lacunosum/Moleculare (O-LM) cells, bistratified cells and long-range projecting cells [4,15,16]. ...
Plasticity of principal cells and inhibitory interneurons underlies hippocampal memory. Bidirectional modulation of somatostatin cell mTORC1 activity, a crucial translational control mechanism in synaptic plasticity, causes parallel changes in hippocampal CA1 somatostatin interneuron (SOM-IN) long-term potentiation and hippocampus-dependent memory, indicating a key role in learning. However, SOM-IN activity changes and behavioral correlates during learning, and the role of mTORC1 in these processes, remain ill-defined. To address these questions, we used two-photon Ca ²⁺ imaging from SOM-INs during a virtual reality goal-directed spatial memory task in head-fixed control mice (SOM-IRES-Cre mice) or in mice with conditional knockout of Rptor (SOM-Rptor-KO mice) to block mTORC1 activity in SOM-INs. We found that control mice learn the task, but SOM-Raptor-KO mice exhibit a deficit. Also, SOM-IN Ca ²⁺ activity became increasingly related to reward during learning in control mice but not in SOM-Rptor-KO mice. Four types of SOM-IN activity patterns related to reward location were observed, “reward off sustained”, “reward off transient”, “reward on sustained” and “reward on transient”, and these responses showed reorganization after reward relocation in control but not SOM-Rptor-KO mice. Thus, SOM-INs develop mTORC1-dependent reward- related activity during learning. This coding may bi-directionally interact with pyramidal cells and other structures to represent and consolidate reward location.
Hippocampal somatostatin-expressing ( Sst ) GABAergic interneurons (INs) exhibit considerable anatomical and functional heterogeneity. Recent single-cell transcriptome analyses have provided a comprehensive Sst -IN subpopulations census, a plausible molecular ground truth of neuronal identity whose links to specific functionality remain incomplete. Here, we designed an approach to identify and access subpopulations of Sst -INs based on transcriptomic features. Four mouse models based on single or combinatorial Cre- and Flp- expression differentiated functionally distinct subpopulations of CA1 hippocampal Sst- INs that largely tiled the morpho-functional parameter space of the Sst -INs superfamily. Notably, the Sst;;Tac1 intersection revealed a population of bistratified INs that preferentially synapsed onto fast-spiking interneurons (FS-INs) and were sufficient to interrupt their firing. In contrast, the Ndnf;;Nkx2-1 intersection identified a population of oriens lacunosum-moleculare INs that predominantly targeted CA1 pyramidal neurons, avoiding FS-INs. Overall, our results provide a framework to translate neuronal transcriptomic identity into discrete functional subtypes that capture the diverse specializations of hippocampal Sst -INs.
Parvalbumin-expressing (PV+) inhibitory interneurons drive gamma oscillations (30–80 Hz), which underlie higher cognitive functions. In this review, we discuss two groups/aspects of fundamental properties of PV+ interneurons. In the first group (dubbed Before Axon), we list properties representing optimal synaptic integration in PV+ interneurons designed to support fast oscillations. For example: [i] Information can neither enter nor leave the neocortex without the engagement of fast PV+ -mediated inhibition; [ii] Voltage responses in PV+ interneuron dendrites integrate linearly to reduce impact of the fluctuations in the afferent drive; and [iii] Reversed somatodendritic Rm gradient accelerates the time courses of synaptic potentials arriving at the soma. In the second group (dubbed After Axon), we list morphological and biophysical properties responsible for (a) short synaptic delays, and (b) efficient postsynaptic outcomes. For example: [i] Fast-spiking ability that allows PV+ interneurons to outpace other cortical neurons (pyramidal neurons). [ii] Myelinated axon (which is only found in the PV+ subclass of interneurons) to secure fast-spiking at the initial axon segment; and [iii] Inhibitory autapses – autoinhibition, which assures brief biphasic voltage transients and supports postinhibitory rebounds. Recent advent of scientific tools, such as viral strategies to target PV cells and the ability to monitor PV cells via in vivo imaging during behavior, will aid in defining the role of PV cells in the CNS. Given the link between PV+ interneurons and cognition, in the future, it would be useful to carry out physiological recordings in the PV+ cell type selectively and characterize if and how psychiatric and neurological diseases affect initiation and propagation of electrical signals in this cortical sub-circuit. Voltage imaging may allow fast recordings of electrical signals from many PV+ interneurons simultaneously.
GABAergic inhibitory neurons are the principal source of inhibition in the brain. Traditionally, their role in maintaining the balance of excitation-inhibition has been emphasized. Beyond homeostatic functions, recent circuit mapping and functional manipulation studies have revealed a wide range of specific roles that GABAergic circuits play in dynamically tilting excitation-inhibition coupling across spatio-temporal scales. These span from gating of compartment- and input-specific signaling, gain modulation, shaping input–output functions and synaptic plasticity, to generating signal-to-noise contrast, defining temporal windows for integration and rate codes, as well as organizing neural assemblies, and coordinating inter-regional synchrony. GABAergic circuits are thus instrumental in controlling single-neuron computations and behaviorally-linked network activity. The activity dependent modulation of sensory and mnemonic information processing by GABAergic circuits is pivotal for the formation and maintenance of episodic memories in the hippocampus. Here, we present an overview of the local and long-range GABAergic circuits that modulate the dynamics of excitation-inhibition and disinhibition in the main output area of the hippocampus CA1, which is crucial for episodic memory. Specifically, we link recent findings pertaining to GABAergic neuron molecular markers, electrophysiological properties, and synaptic wiring with their function at the circuit level. Lastly, given that area CA1 is particularly impaired during early stages of Alzheimer’s disease, we emphasize how these GABAergic circuits may contribute to and be involved in the pathophysiology.
The CA1 region of the hippocampus is one of the most studied regions of the rodent brain, thought to play an important role in cognitive functions such as memory and spatial navigation. Despite a wealth of experimental data on its structure and function, it can be challenging to reconcile information obtained from diverse experimental approaches. To address this challenge, we present a community-driven, full-scale in silico model of the rat CA1 that integrates a broad range of experimental data, from synapse to network, including the reconstruction of its principal afferents, the Schaffer collaterals, and a model of the effects that acetylcholine has on the system. We have tested and validated each model component and the final network model, and made input data, assumptions, and strategies explicit and transparent. The flexibility of the model allows scientists to address a range of scientific questions. In this article, we describe the methods used to set up simulations that reproduce and extend in vitro and in vivo experiments. Among several applications in the article, we focus on theta rhythm, a prominent hippocampal oscillation associated with various behavioral correlates and use our computer model to reproduce and reconcile experimental findings. Finally, we make data, code and model available through the hippocampushub.eu portal, which also provides an extensive set of analyses of the model and a user-friendly interface to facilitate adoption and usage. This neuroscience community-driven model represents a valuable tool for integrating diverse experimental data and provides a foundation for further research into the complex workings of the hippocampal CA1 region.
Hippocampal somatostatin-expressing ( Sst ) GABAergic interneurons (INs) exhibit considerable anatomical and functional heterogeneity. Recent single cell transcriptome analyses have provided a comprehensive Sst -IN subtype census, a plausible molecular ground truth of neuronal identity whose links to specific functionality remain incomplete. Here, we designed an approach to identify and access subpopulations of Sst -INs based on transcriptomic features. Four mouse models based on single or combinatorial Cre- and Flp- expression differentiated functionally distinct subpopulations of CA1 hippocampal Sst- INs that largely tiled the morpho-functional parameter space of the Sst -INs superfamily. Notably, the Sst;;Tac1 intersection revealed a population of bistratified INs that preferentially synapsed onto fast-spiking interneurons (FS-INs) and were both necessary and sufficient to interrupt their firing. In contrast, the Ndnf;;Nkx2-1 intersection identified a population of oriens lacunosum-moleculare (OLM) INs that predominantly targeted CA1 pyramidal neurons, avoiding FS-INs. Overall, our results provide a framework to translate neuronal transcriptomic identity into discrete functional subtypes that capture the diverse specializations of hippocampal Sst -INs.
Significance statement
GABAergic interneurons are important regulators of neuronal activity. Recent transcriptome analyses have provided a comprehensive classification of interneuron subtypes, but the connections between molecular identities and specific functions are not yet fully understood. Here, we developed an approach to identify and access subpopulations of interneurons based on features predicted by transcriptomic analysis. Functional investigation in transgenic animals revealed that hippocampal somatostatin-expressing interneurons ( Sst -INs) can be divided into at least four subfamilies, each with distinct functions. Most importantly, the Sst;;Tac1 intersection targeted a population of bistratified cells that overwhelmingly targeted fast-spiking interneurons. In contrast, the Ndnf;;Nkx2-1 intersection revealed a population of oriens lacunosum-moleculare interneurons that selectively targeted CA1 pyramidal cells. Overall, this study reveals that genetically distinct subfamilies of Sst -INs form specialized circuits in the hippocampus with differing functional impact.
γ-Amlnobutyric acid (GABA) can have a biphasic effect on membrane
potential when applied to certain central nervous system
neurones1-3. Typically with iontophoresis, this effect is a
negative-positive sequence of deflections and both phases are associated
with an increase in conductance. However, iontophoretic application of
neurotransmitter substances can produce effects even in cases where
ordinary synaptic transmitter release is impossible; for example, on
dorsal root ganglion cells. Thus, it is important in assessing the
physiological relevance of the depolarising GABA response to determine
if similar actions are produced synaptically. In addition, although the
depolarising aspect of the GABA response has been attributed to
activation of dendritic receptors, this identification has remained
uncertain. Here, we have used the hippocampal slice preparation to
demonstrate that a biphasic inhibitory response results from
orthodromic, but not antidromic electrical stimulation in the presence
of anaesthetic concentrations of pentobarbital. (For the sake of
brevity, we use the term `biphasic inhibitory postynaptic potential
(i.p.s.p.)' to describe the response complex, although it seems likely
that a combination of two distinct events, rather than two parts of a
single event, occurs.) GABA antagonists and tetrodotoxin (TTX) were used
to show that the depolarising phase is due to synaptically released GABA
acting on dendritic sites in the CA1 region. Both negative and positive
phases are associated with a large conductance increase, and both block
cell firing, indicating their inhibitory nature. The ionic mechanism of
the depolarising phase is not fully understood but seems to involve a
chloride conductance increase. Although the functional significance of
the depolarising sign of the i.p.s.p. is not clear, the dendritic
location would provide very effective control over dendritic
excitability.
1. The interaction between inhibitory interneurons and cortical pyramidal neurons was studied by use of computer simulations to test whether inhibitory interneurons could assist in phase-locking postsynaptic cells. Two models were used: a simplified model, which included only 3 membrane channels, and a detailed 11-channel model. 2. The 11-channel model included most of the ion channels known to be present in neocortical pyramidal neurons as well as calcium diffusion and other membrane mechanisms. The kinetics for the channels were obtained from voltage-clamp studies in a variety of preparations. The parameters were then adjusted to produce repetitive bursting similar to that seen in some cortical pyramidal cells entrained during visual stimulation. 3. Phase-locking to a train of inhibitory postsynaptic potentials (IPSPs) located on or near the soma was observed in the 3-channel model cell subjected to random synaptic bombardment. In the 11-channel model, phase-locking due to multiple IPSPs was compared with phase-locking due to multiple excitatory postsynaptic potentials (EPSPs). Phase-locking began to occur when 20% of the IPSPs (20/100) or 40% of the EPSPs (4,000/10,000) were synchronized. The exact percentages differed with different 11-channel models, but either EPSPs or IPSPs would generally produce entrainment with approximately 40% synchronization. Thus 40 inhibitory boutons had an effect equivalent to 4,000 excitatory boutons in producing phase-locking. 4. Phase-locking with IPSPs in these models was possible because the IPSPs could cause either an increase or a decrease in firing rate over a limited range. The IPSPs served a modulatory role, increasing the rate of firing in some cases and decreasing it in others, depending on the state of the cell. 5. We examined frequency entrainment by IPSPs. In the 3-channel model, frequency entrainment of a postsynaptic cell was observed with a rapid train of strong (20-100 nS), brief, compound IPSPs. A 40-Hz compound IPSP train of 60 nS entrained cells having initial firing rates between 32 and 47 Hz. Below this range, cells could be partially entrained. Above the range, entrainment would fail. Frequency entrainment in the 3-channel model generally occurred on the first cycle after onset of the IPSPs. 6. Phase-locking and frequency entrainment were less robust in the 11-channel model. This was partly because bursts rather than individual spikes were being entrained. A 40-Hz, 90-nS compound IPSP train entrained a model cell upward from 34 Hz. Downward frequency entrainment also occurred.(ABSTRACT TRUNCATED AT 400 WORDS)
We examine the effect of inhibition on the axon initial segment (AIS) by the chandelier (axoaxonic) cells, using a simplified compartmental model of actual pyramidal neurons from cat visual cortex. We show that within generally accepted ranges, inhibition at the AIS cannot completely prevent action potential discharge: only small amounts of excitatory synaptic current can be inhibited. Moderate amounts of excitatory current always result in action potential discharge, despite AIS inhibition. Inhibition of the somadendrite by basket cells enhances the effect of AIS inhibition and vice versa. Thus the axoaxonic cells may act synergistically with basket cells: the AIS inhibition increases the threshold for action potential discharge, the basket cells then control the suprathreshold discharge.
Chandelier or axo-axonic cells (AACs) are specialized interneurons terminating on the axon initial segments of pyramidal neurons. Two AACs have been localized by Golgi impregnation, one in the CA1 region of the hippocampus and one in the visual cortex of cat, for structural analysis and for the identification of their transmitter. They had 323 and 268 terminal bouton rows, respectively, probably making synapses with an equal number of initial segments. The distribution of the dendrites of the hippocampal cell was strikingly similar to that py pyramidal ceells suggesting a similar input. Using an antiserum to GABA and postenbedding GABA-immunocytochemistry, developed for Golgi-impregnated neurons, both cells were found to be GABA-immunoreatiive. The strategic location of their synapses and the presence of GABA in AACs suggest that in normal cortical tissue they play a major role in GABA-mediated inhibition.
The enzyme that synthesizes the neurotransmitter γ-aminobutyric acid (GABA), glutamic acid decarboxylase (GAD), has been immunocytochemically localized in the somata and dendrites of certain neurons in rat cerebellum and Ammom's horn following colchicine injections into these two brain regions. In the cerebellum. GAD-positive reaction product was observed in the somata and proximal dendrites of Purkinje, Golgi II, basket and stellate neurons. Occasional staining of the proximal portions of axons was also observed in these colchicine-injected preparations. None of the somata or dendrites of these same cell types exhibited reaction product in preparations that were not pretreated with colchicine, although the axon terminals of these neurons were GAD-positive. In Ammon's horn, the somata of a few cells that are classified as probable basket and other short-axon neurons contained detectable concentrations of GAD in preparations that were not pretreated with colchicine. However, following colchicine injections into the Ammon's horn, there was approximately a five-fold increase in the number of GAD-positive somata of basket and other short-axon neurons. There was also a substantial increase in the extent of dendritic staining exhibited by these neurons. Control injections of saline and lumicolchicine produced the same results as those observed in preparations which were not pretreated with colchicine. Thus, the results from the control injections indicate that the increases in somal and dendritic staining are due to a colchicine-mediated inhibition of the somatofugal transport of GAD rather than to a non-specific effect of the drug and/or the injection procedure.
1. Whole-cell patch-clamp techniques were used to record from dentate gyrus granule cells in adult rat brain slices when N-methyl-D-aspartate (NMDA) and non-NMDA type glutamate receptors were blocked by D-2-amino-5-phosphonovaleric acid (D-AP5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), respectively. Spontaneous inhibitory postsynaptic currents (sIPSCs), each presumably due to vesicular release of gamma-aminobutyric acid (GABA), selectively activated GABAA-type receptors. None of the individual sIPSCs showed a slow-onset potassium current characteristic of GABAB receptor activation. 2. In contrast, stimulation in the molecular layer with a bipolar stimulating electrode or bath application of the convulsant drug 4-aminopyridine (4-AP, 10-30 microM) elicited fast GABAA IPSCs followed by slower outward currents that were sensitive to the selective GABAB antagonist CGP 35348 (0.1-1 mM) and that reversed polarity near the potassium equilibrium potential. 3. CGP 35348 (0.5-1 mM) or the GABAB agonist (-)baclofen (1 microM) had no significant effect on the frequency or average amplitude of sIPSCs. However, either bath application of (-)baclofen (1 microM) or a preceding conditioning stimulus caused large reductions in the amplitude of stimulus-evoked IPSCs, suggesting a strong GABAB-mediated presynaptic inhibition of stimulus-evoked GABA release. 4. We conclude that under normal conditions spontaneous transmitter release does not activate GABAB receptors in dentate gyrus slices. These findings are consistent with either of two general possibilities. Separate groups of interneurons with different basal firing rates may selectively form GABAA and GABAB synapses.(ABSTRACT TRUNCATED AT 250 WORDS)
1. Stimulation of the perforant path in the outer molecular layer of the adult rat dentate gyrus produced a depolarizing post-synaptic potential (DPSP) in granule cells when recorded using whole-cell techniques in the standard hippocampal slice preparation at 34 degrees C. The postsynaptic currents (PSCs) contributing to the DPSP were analyzed using specific receptor antagonists in current- and voltage-clamp recordings. 2. The DPSP reversal potential was dependent on the intracellular chloride concentration, and the amplitude of the DPSP was increased 55% after perfusion of the gamma-aminobutyric acid-A (GABAA) receptor antagonist bicuculline methiodide (BMI). The GABAA receptor-mediated PSC reversed at -66 mV, which was 19 mV positive to the resting membrane potential (-85 mV) but hyperpolarized relative to action potential threshold. At -35 mV, the GABAA PSC had a latency to peak of 12.9 ms after the stimulus and decayed monoexponentially with an average time constant of 23.4 ms. 3. The component of the PSC blocked by the Quis/AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) had a latency to peak of 7.1 ms and decayed monoexponentially with a time constant of 9.9 ms at -35 mV. The N-methyl-D-aspartate (NMDA) receptor-mediated PSC, which was blocked by D-amino-5-phosphonovaleric acid (D-AP5), had a waveform that was similar to the GABAA PSC: the latency to peak was 16 ms and the decay was monoexponential with a time constant of 24.5 ms at -35 mV. 4. The ratio of the peak PSCs mediated by GABAA, Quis/AMPA, and NMDA receptors measured at -35 mV with cesium gluconate electrode solutions was 1:0.2:0.1. This ratio was essentially constant over the range of stimulus intensities that produced compound PSC amplitudes of 80-400 pA. 5. Measured at its reversal potential, the GABAA receptor-mediated postsynaptic conductance (GGABA-A) decreased the peak DPSP amplitude by 35%, shunted 50% of the charge transferred to the soma by the excitatory PSC, and completely inhibited the NMDA receptor-mediated component of the DPSP. 6. Simultaneous stimulation of presynaptic fibers from both the perforant path and interneurons results in a large depolarizing GGABA-A that inhibits the granule cell by shunting the excitatory PSCs. As predicted by models of shunting, the similar kinetics of the GABAA and NMDA PSCs leads to particularly effective inhibition of the NMDA PSC. The more rapid Quis/AMPA PSC is less affected by the GGABA-A, so that granule cell excitation under these conditions is primarily due to Quis/AMPA receptor activation.
Most neurons have inhibitory synapses both "proximally" near the spike-initiating zone and "distally" on dendrites. Although distal inhibition is thought to be an adaptation for selective inhibition of particular dendritic branches, another important distinction exists between proximal and distal inhibition. Proximal inhibition can attenuate excitatory input absolutely so that no amount of excitation causes firing. Distal inhibition, however, inhibits relatively; any amount of it can be overcome by sufficient excitation. These properties are used as predicted in the circuit-mediating crayfish escape behavior. Many neuronal computations require relative inhibition. This could partly account for the ubiquity of distal inhibition.
With the use of in vitro preparations and sophisticated electrophysiological recording techniques, we are beginning to understand in great detail how a wide variety of putative neurotransmitters alter neuronal excitability. Many of these recent results have changed our previous conceptualization of excitatory and inhibitory synaptic transmission. For example, as we reviewed, it is now clear that in many regions of brain, EPSPs, although mediated by a single neurotransmitter, glutamate, are composed of two components that subserve distinct functions. Similarly, the elucidation of the different properties of GABA(A) and GABA(B) receptors has demonstrated that inhibition in the brain is also not a single process but most likely has at least two components. It is also now well established that many neurotransmitter receptors are coupled to second messengers, the activation of which in turn alters ion channel activity. Many of these second messenger systems modulated voltage-dependent channels, resulting not in a simple membrane depolarization or hyperpolarization but rather an effect that depends on the membrane potential of the cell, a property that is constantly changing in both the spatial and temporal domains. This type of modulation can result in drastic changes in the cells' input-output properties. By being coupled to the same second messenger system or G protein, distinct neurotransmitter receptors may induce the same electrophysiological response. Figure 1 shows this convergence of action of various neurotransmitter receptors onto two types of ion channels, an inwardly rectifying K+ channel (I(K)) and a Ca2+-activated K+ channel (I(AHP). In many cases this convergence can be seen in a single type of neuron. Thus GABA(B), 5-HT(1a), A1, and probably SS receptors are coupled to the same K+ channel in CA1 hippocampal pyramidal cells. In locus coeruleus neurons, μ-opioid, α2-noradrenergic, SS, and probably GABA(B) receptors all activate the same K+ conductance. In lateral parabrachial neurons, μ-opioid, M2, and GABA(B) receptors increase the same K+ conductance. Finally, in substantia nigra neurons, GABA(B) and D2 receptors, activate the same K+ conductance. For I(AHP), all of the neurotransmitters listed act on CA1 hippocampal pyramidal cells and converge onto the same K+ conductance. The receptor subtype of the 5-HT effect and the coupling mechanism for the 5-HT and ACh effects have not been clearly established. In addition to convergence, the use of G proteins and second messenger systems to mediate the actions of neurotransmitters may offer additional advantages. For example, parts of a cell spatially segregated from a given receptor could be affected by activation of that receptor via its coupling to diffusible second messengers. Actions mediated by second messengers might be long lasting and more subject to subtle, long-term modulations. It is also likely that neurotransmitters that activate receptors coupled to G proteins and second messengers have functionally important biochemical effects in addition to directly modulating ion channels. Although different neurotransmitters often appear to have similar electrophysiological actions, they can also be shown to have quite distinct actions, primarily because they activate a variety of subtypes of receptors that are coupled to distinct G proteins and second messengers. Thus the same neurotransmitter can have distinct actions on different cells or different parts of the same cell because of the differential distribution of receptor subtypes. A corollary of this is that if a given neurotransmitter is known to have different actions in different brain regions, it most likely is because the receptors it is activating are distinct. Figures 2-4 show this striking divergence of action for five different neurotransmitters. Glutamate probably activates five types of receptor. Three of these, the AMPA, the kainate, and the NMDA receptors, cause a rapid increase in cation conductance without an intervening coupling molecule. A subtype of quisqualate receptor, which differs pharmacologically from the quisqualate receptor linked to ion channels (now generally referred to as the AMPA receptor), is coupled to PI turnover. The physiological consequence of activating this receptor has yet to be observed in CNS neurons. Activation of the AP4 receptor causes a presynaptic inhibition in a number of pathways within the CNS, perhaps by decreasing Ca2+ entry into the nerve terminal. There is only limited data that glutamate can act on this presynaptic receptor. γ-Aminobutyric acid activates two types of receptors. The GABA(A) recepors rapidly open Cl- channels without an intervening coupling molecule.
1. Synaptically connected neurones were identified in the granule cell layer of slices of 17- to 21-day-old rat hippocampus. Whole-cell current recording using the patch-clamp technique revealed synaptic currents ranging from less than 10 to 200 pA in symmetrical Cl- conditions, at a holding potential of -50 mV. These currents were blocked by 2 microM-bicuculline, indicating that they result from the activation of postsynaptic gamma-aminobutyric acid receptor (GABAA-receptor) channels. 2. Addition of tetrodotoxin (TTX, 1 microM) resulted in the loss of most currents of more than 40 pA in amplitude. Currents which disappeared after TTX treatment were assumed to be the result of spontaneous presynaptic action potentials. The currents seen in the absence of TTX are referred to as spontaneously occurring inhibitory postsynaptic currents (IPSCs); those remaining in the presence of TTX were defined as miniature IPSCs. 3. Similar currents were observed when recording in the whole-cell configuration while extracellular stimulation was applied to a nearby neurone. These currents were also completely blocked by 2 microM-bicuculline and by 0.5 microM-TTX. They were thus defined as stimulus-evoked IPSCs. 4. The half rise time of both miniature and stimulus-evoked IPSCs was fast (less than 1 ms). The time course of decay of both miniature IPSCs and stimulus-evoked IPSCs could be well fitted with the sum of two exponentials. At a membrane potential of -50 mV, the mean decay time constants of the two components were 2.0 +/- 0.38 and 54.4 +/- 18 ms (mean +/- S.D.) for miniature IPSCs (six cells) and 2.2 +/- 1.3 and 66 +/- 20 ms (three cells) for stimulus-evoked IPSCs. 5. Stimulus-evoked IPSCs varied in amplitude from less than ten to hundreds of picoamperes. In eight of eleven cells histograms of IPSC amplitudes showed several clear peaks which, when fitted with the sum of Gaussian curves, were found to be equidistant. This is consistent with the view that stimulus-evoked IPSC amplitudes vary in a quantal fashion. The quantal size varied between 7 and 20 pA, at a membrane potential of -50 mV. 6. Decreasing the Ca2+ and increasing the Mg2+ concentration in the extracellular solution decreased the number of peaks in the IPSC amplitude histogram but did not affect the size of the quantal event.(ABSTRACT TRUNCATED AT 400 WORDS)